I have been discussing with a friend of mine about making up a circuit for switching between batteries 9v and 18v.
Essentially the switch will be 4 way with the following options.
1. 2 x 9v in Series - 18v mod.
2. 2 x 9v in Parallel; 9v.
3. 1st Battery 9v.
4. 2nd Battery 9v.